In recent years, miniaturization and power saving of transmission apparatuses for optical communications have been pursued. At present, a coplanar electrode structure, for example, is used in an optical waveguide device as a transmission apparatus. The coplanar electrode structure may control a refractive index based on an electric field supplied to the optical waveguide. However, a driving voltage is generally increased in order to provide control of the refractive index.
In addition to the coplanar electrode structure, a structure for concentrating lines of electric force on the optical waveguide by forming an electrode right under the optical waveguide has been proposed in Japanese Laid-Open Patent Publication No. 11-326853, for example.
According to a technique described in Japanese Laid-Open Patent Publication No. 11-326853, to epitaxially grow a lithium niobate (LiNbO3:LN) thin-film, ZnO (a conductive crystal material) is employed as an electrode material formed directly under the waveguide. The conductive crystal material of the electrode matches a lattice constant of an LN crystal. However, a conductive crystal material such as ZnO has a resistance value that is expected to be higher than that of a metal in a low frequency area. Further, the resistance value of the conductive crystal material is expected to be even higher than that of the metal in a high frequency area.
Accordingly, using a conductive crystal material such as ZnO as an electrode driving at high-frequency voltage may cause large losses affecting the effectiveness of the electrode, which results in an increase of a related driving voltage. In general, the resistance value of the metal is relatively low compared to that of the conductive crystal material. However, in a technique described in Japanese Laid-Open Patent Publication No. 11-326853, the metal may not be used in place of the conductive crystal material at least because of the issue of matching the lattice constant.